Hybrid vehicle transmission shift control for electric machine thermal management

Abstract

A system and method for controlling a hybrid vehicle having an engine, a traction motor, and an automatic step-ratio transmission having a plurality of selectable discrete gear ratios, a torque converter, and sharing a common cooling fluid or oil with the traction motor, include a controller configured to shift the automatic step-ratio transmission to neutral in response to vehicle speed being below a threshold with the engine idling while temperatures of at least two of: the traction battery, the electric machine, and the transmission are above associated thresholds to reduce heat generation by the torque converter due to oil shear with the torque converter stalled. The controller may shift the transmission to drive in response to release of a brake pedal. A neutral shift may also be performed in response to high-voltage accessory loads or on-board generator loads exceeding a threshold.

What is claimed is: 
     
       1. A vehicle comprising:
 an engine; 
 an electric machine selectively coupled by a first clutch to the engine; 
 a step-ratio transmission selectively coupled by a second clutch to the electric machine, the step-ratio transmission and the electric machine configured to share a common cooling fluid; and 
 a controller programmed to shift the step-ratio transmission to neutral when the engine is idling, vehicle speed is below a speed threshold, and at least one of a temperature of the electric machine exceeds a first temperature threshold and a temperature of the cooling fluid exceeds a second temperature threshold, and wherein the controller is further programmed to shift the step-ratio transmission to neutral in response to a vehicle high-voltage accessory load exceeding a corresponding threshold. 
 
     
     
       2. The vehicle of  claim 1  wherein the step-ratio transmission comprises a torque converter and the second clutch comprises a torque converter lock-up clutch. 
     
     
       3. The vehicle of  claim 2  wherein the controller is further programmed to shift the step-ratio transmission to drive in response to release of a brake pedal. 
     
     
       4. The vehicle of  claim 1  wherein the electric machine and the step-ratio transmission are positioned within a common housing. 
     
     
       5. The vehicle of  claim 1  wherein the controller is further programmed to shift the step-ratio transmission to neutral in response to depression of a brake pedal. 
     
     
       6. The vehicle of  claim 1  further comprising a high-voltage battery electrically coupled to the electric machine, wherein the controller is further programmed to shift the step-ratio transmission to neutral in response to temperature of the high-voltage battery exceeding a corresponding battery temperature threshold. 
     
     
       7. A system comprising:
 an electric machine selectively coupled by an upstream clutch to an engine and selectively powered by a traction battery to propel the vehicle; 
 an automatic step-ratio transmission having a plurality of selectable discrete gear ratios and a torque converter, the transmission selectively coupled to the electric machine; and 
 a controller configured to:
 shift the automatic step-ratio transmission to neutral in response to vehicle speed being below a threshold with the engine idling while temperatures of at least two of: the traction battery, the electric machine, and the transmission are above associated thresholds. 
 
 
     
     
       8. The system of  claim 7  wherein the controller is further configured to shift the automatic step-ratio transmission to drive in response to release of a vehicle brake pedal or depression of a vehicle accelerator pedal. 
     
     
       9. The system of  claim 8  wherein the electric machine and the automatic step-ratio transmission are both cooled by a shared volume of transmission oil. 
     
     
       10. The system of  claim 7  wherein the controller is further configured to shift the automatic step-ratio transmission to neutral in response to a high-voltage accessory load exceeding a corresponding threshold. 
     
     
       11. The system of  claim 7  wherein the controller is further configured to shift the automatic step-ratio transmission to neutral in response to depression of a vehicle brake pedal. 
     
     
       12. The system of  claim 11  wherein the controller is further configured to shift the automatic step-ratio transmission to neutral in response to the vehicle being stopped. 
     
     
       13. A method for controlling a vehicle having an engine, an electric machine coupled to a traction battery, and a step-ratio transmission with a torque converter coupled to the electric machine, the method comprising, by a vehicle controller:
 shifting the step-ratio transmission to neutral while temperatures of at least two of the electric machine, the traction battery, and the step-ratio transmission exceed associated temperature thresholds in response to the vehicle stopping with the engine idling; and 
 shifting the step-ratio transmission from neutral to drive in response to at least one of: releasing a vehicle brake pedal; and depressing a vehicle accelerator pedal, while the vehicle is stopped and the engine is idling. 
 
     
     
       14. The method of  claim 13  wherein the electric machine and the step-ratio transmission share a common cooling fluid. 
     
     
       15. The method of  claim 14  wherein the vehicle controller shifts the step-ratio transmission to neutral in response to vehicle speed decreasing below an associated speed threshold. 
     
     
       16. The method of  claim 14  wherein the vehicle controller determines the engine is idling based on the vehicle accelerator pedal being fully released. 
     
     
       17. The method of  claim 14  wherein the vehicle controller shifts the step-ratio transmission to neutral further in response to vehicle high-voltage accessory load exceeding a corresponding threshold. 
     
     
       18. The method of  claim 17  wherein the vehicle controller shifts the step-ratio transmission to neutral further in response to depression of the brake pedal.